The present invention is related to two known types of construction called “spacer band” and “platform”. In the spacer band type of construction, shown in FIG. 1A, the blade aerofoils 1 are fixed to radially inner and outer structural members 2, 3 by means of inner and outer annular “bands” 4, 5, which are rolled from flat strip. Holes 6 are cut in the bands 4, 5 (e.g., by laser) to match the cross-section of the aerofoil shape, the ends of the aerofoils then being inserted into the holes and fillet welded into place. The bands are in turn welded to the inner and outer members 2, 3 to form the steam passage wall, or so-called “port wall”.
An advantage of spacer band construction is that it involves a relatively small amount of machining of the blades compared to some other types of construction used in steam turbines. For example, in the so-called “platform” type of diaphragm construction, see FIG. 1B, the ends of the aerofoils 7 are integral with radially inner and outer “platforms” 8, 9, the aerofoils and platforms being machined from solid. A complete annulus of static blades is built up by assembling successive combined aerofoil/platform components 10 between inner and outer rings and welding the platforms to the rings. To illustrate the assembly process, an adjacent blade shape is shown in dashed lines. When the assembly is finished, the inner and outer platforms 8, 9 form the inner and outer port walls of the diaphragm.
This form of construction has performance advantages compared to the spacer band type, but has much higher manufacturing costs due to the extra material and machining involved in producing combined aerofoil/platform components 10. However, for some types of use—retrofitting of existing turbines, in particular—the performance benefit of the platform type of construction makes it the most economic solution despite its extra cost. This performance benefit arises from the fact that platform construction enables an aerodynamically advantageous fillet radius 11 in the corner between the aerofoil and the port wall, a smoother port wall and better maintenance of circularity in the port wall during operation of the turbine.
It is an object of the present invention to provide a type of diaphragm construction which has the good performance characteristics of the above platform type of construction, while being more economical to manufacture.